Thin film electroluminescent (TFEL) display panels offer several advantages over other display technologies such as cathode ray tubes and liquid crystal displays. Compared with cathode ray tubes, TFEL display panels require less power, provide a larger viewing angle, and are much thinner. Compared with liquid crystal displays, TFEL display panels have a larger viewing angle, do not require auxiliary lighting and can have a larger display area.
FIG. 1 shows a prior art TFEL display panel. The TFEL display has a glass panel 10, a plurality of transparent electrodes 12, a first layer of dielectric 14, a phosphor layer 16, a second dielectric layer 18, and a plurality of metal electrodes 20 perpendicular to the transparent electrodes 12. The transparent electrodes 12 are typically indium-tin oxide (ITO) and the metal electrodes 20 are typically aluminum. The dielectric layers 14, 18 protect the phosphor layer 16 from excessive dc currents. When an electrical potential, such as about 200 V, is applied between the transparent electrodes 12 and the metal electrodes 20, electrons tunnel from one of the interfaces between the dielectric layers 14, 18 and the phosphor layer 16 into the phosphor layer where they are rapidly accelerated. The phosphor layer 16 typically comprises ZnS doped with Mn. Electrons entering the phosphor layer 16 excite the Mn causing the Mn to emit photons. The photons pass through the first dielectric layer 14, the transparent electrodes 12, and the glass panel 10 to form a visible image.
Speed and brightness uniformity of the electroluminescent display depend critically on the ITO line resistance, particularly for large area displays. Even with integrated ITO lines, a zebra pattern brightness contrast occurs due to high resistance. Therefore, to achieve higher conductivity of the transparent electrodes in the electroluminescent display panel, structures have been developed in which the low conductivity ITO electrodes were augmented by buses of thick, narrow, high conductivity metals. In the most common augmented ITO panel the metal assist structure is on top of the ITO electrode and adjacent the overlaying dielectric layer.
During the manufacture of TFEL displays it is necessary to anneal the structure after applying the stack of a dielectric layer, phosphor layer and dielectric layer over the transparent electrodes and metal substrate. In those displays which have metal assist structures between the ITO electrode and the dielectric layer, migration occurs between the metal assist structure and the dielectric layer. Such migration is generally considered undesirable because this migration can result in increased resistance of the metal assist structure.
O. J. Gregory et al. discuss the migration problem in their article "Fabrication of High Conductivity Transparent Electrodes with Trenched Metal Bus Lines," Journal of the Electro Chemical Society, Vol. 138, No. 7, July, 1991. Their solution to this problem is TFEL display which utilizes augmented ITO electrodes in which the metal assist structure for each electrode is etched into the glass substrate. Each ITO electrode is then deposited over a metal conductor. To make the panel disclosed by Gregory one must etch the glass substrate to provide paths for the metal assists and then the metal must be deposited in those grooves which have been cut into the glass. Etching the glass adds an additional step to the production. Additionally, making such grooves can cause the glass to crack or be more likely to fail in those areas. Therefore, this structure is not practical.
Thus, there is a need for an improved electroluminescent display panel which utilizes a metal assist structure, and which can be annealed without causing migration between the metal assist structure and a dielectric layer. Moreover, such an improved electroluminescent display must be easy to manufacture, preferably with conventional manufacturing techniques while adding no significant cost to the electroluminescent display.